The present invention concerns improvements relating to compressed non-asbestos sheets designed for use as gasket materials.
Compressed asbestos sheets are currently available and are used in a wide range of industries, such as shipbuilding, chemical engineering, and in the production of autmobiles and various equipment.
The compressed asbestos sheet is a sheet-like object or article prepared by mixing asbestos fibers, which serve as a base, with rubber, which acts as a binder, and pressing the resultant mixture under pressure into an intimate and uniform cardboard. Depending upon the properties demanded, 60 to 80% of 3 to 7 class asbestos is mixed with natural or synthetic rubber which is dissolved in an organic solvent in quantities of 10 to 20% calculated as solid matter together with other rubber chemicals and fillers. The mixture thus obtained is then laminated on a heated roll.
The asbestos used in the aforesaid sheet is a certain kind of mineral that is increasingly unavailable since its resources are being exhausted, along with the fact that expliotation and transportation costs are increasing. A severe problem also arises in connection with the adverse influence of asbestos on the human body, thus imposing additional restrictions upon the use of asbestos all over the world.
As mentioned above, the compressed asbestos sheet has its major portion composed of asbestos, and involves all the problems stemming from asbestos. For this reason, there is now a strong demand for the development of compressed asbestos-free (non-asbestos) sheets.
A variety of compressed sheets free from asbestos and consisting of other fibers have been proposed for the purpose of providing a solution to the aforesaid problems.
For example, the specification of pre-published Japanese Patent Application No. 86,659/1976 discloses a sheet including as a base inorganic fibers such as glass fibers, ceramic fibers, rock wool, and a heat-resistant inorganic material. This sheet is prepared in the same manner as mentioned in conjunction with the production of the compressed asbestos sheet. However, since this sheet has inferior physical properties, e.g., a tensile strength less than that of the ordinary asbestos sheet by a factor of about 1/3-1/4, it has not been put to practical use as yet.
The specification of pre-published Japanese Patent Application No. 29,658/1976 teaches a gasket material in which a mixture of asbestos and carbon fibers is used as base fibers; and a synthetic rubber latex is used as a binder. The addition of carbon fibers is intended to make an improvement in the heat resistance of the gasket material. However, with the inclusion of asbestos, the problems resulting from its use are also present.
In addition, the specification of pre-published Japanese Patent Application No. 25,824/1975 describes an inorganic fibrous sheeting prepared in a sheet making system, in which non-metallic inorganic fibers and metal fibers are used as a base, and a rubber latex is used as a binder. This sheet material was intended to enhance the heat resistance and sealing properties by mixing the asbestos-containing inorganic fibers with the metal fibers. Since this material is prepared in a sheet making system with the use of a rubber latex binder, however, it does not have the mechanical strength of the compressed asbestos sheet.
The inorganic fibrous sheet, which is prepared in a sheet making system with the use of a rubber latex, as mentioned above, is generally called a beater sheet. The compressed sheet, which is obtained by a laminating method carried out on a heated roll with the use of rubber dissolved in an organic solvent, is identical to the inorganic fibrous sheet in composition. However, they are distinguished from each other by their difference in structure, as will be explained below.
The beater sheet is made by forming the starting material into a sheet while it is in a moist state, and thereafter removing the moisture therefrom by simultaneously heating and vulcanizing. This causes a relatively large number of voids to be left in the sheet after the moisture has been removed.
The compressed sheet is made by pressing and laminating the starting material with the evaporation of a solvent while subjecting it to vulcanization at the same time, thus keeping its void content at an extremely low level. Even though the beater sheet is treated on, e.g., a heated roll, to reduce its void content, it is impossible to attain the low level of voids of the compressed sheet.
In the beater sheet which uses a rubber latex as a binder, the rubber is apt to exist on the surface of the fibers in the form of grains. As a result, the adhesion of the rubber to the fibers is less in comparison to other types of compressed sheets wherein the rubber is dispersed uniformly over the surface of the fibers, such as the type of compressed sheet that makes use of a rubber dissolved in a solvent. Thus, the rubber latex does not act as an efficient binder in the beater sheet.
For the reasons set forth above, the beater sheet is considerably poorer in the physical properties, such as mechanical strength, and sealing properties that are needed for gaskets or similar parts, in comparison to the compressed sheet. Therefore, unlike the compressed sheet, the beater sheet is not suited for the production of parts that are used under stressful conditions.